Platelet activating factor has now been identified by others as acetyl-glyceryl-ether-phosphoryl-choline (AGEPC). This low molecular weight lipid is released by a variety of cells including human leukocytes, sensitized rabbit basophils and rat peritoneal macrophages. This molecule aggregates rabbit and human platelets and causes them to secrete serotonin and other platelet constituents. It has been proved to be involved in anaphylaxis when injected intravenously into normal or sensitized rabbits. AGEPC is an important mediator during allergic reactions in animals and the present research is investigating the possibility that AGEPC or PAF also is an important mediator during allergic reactions in man. Previous failures to isolate PAF in vivo during antigen induced anaphylaxis were due to the presence of a serum factor that inactivates the biologic activity of PAF. This inactivation occurs within seconds after mixing PAF with plasma or serum in vitro or after the release of PAF in vivo during rabbit anaphylaxis. This factor is inactivated when exposed to pH 3.0 for five minutes. A factor similar to the acid labile factor (ALF) in rabbit plasma has also been found in human plasma during the past year. It is associated with beta-lipoprotein and we have been able to disassociate ALF activity from beta-lipoprotein by treating a low density lipoprotein fraction with tetramethyl urea. Research during the coming year will be directed towards further isolation of ALF, and an attempt to develop a radioimmunoassay for it. We then propose to study the relationship between serum ALF levels and the generation of human PAF in vitro after adding specific antigen to whole blood from allergic persons; to look for human PAF in vivo during experimentally induced asthma after inhalation challenge with antigens; to look for low levels of ALF in serum from patients with lung diseases associated with immune disorders; and patients with thrombocytopenias. Further, during the past year we have found PAF in human saliva and an attempt will be made to learn the significance of its presence.